Electrophysiological analysis of the tecto-olivocerebellar (lobule VII) projection in the rat

Brain Research, 340 (1985) 369-372

369

Elsevier B R E 20979

Electrophysiological analysis of the tecto-olivocerebellar (Iobule VII) projection in the rat TADASHI AKAIKE

Department of Physiology, Nagoya University School of Medicine, Nagoya 466 (Japan) (Accepted March 19th, 1985)

Key words: cerebellum - - superior colliculus - - inferior olive - - lobule VII - - albino rat

In albino rats the superior colliculus was stimulated and its evoked potentials were explored throughout the posterior vermis of thc cerebellum. Climbing fiber responses were identified only in lobule VII, ipsilaterally 1.2-1.6 m m wide. In the medial accessory olive, subnucleus c, in the contralateral side both antidromically evoked potentials from lobule VII and orthodromically evoked potentials from the superior colliculus were recorded. This evidence suggests that they are tecto-olivocerebellar projections.

There is ample evidence that neurons in the posterior vermis of lobule VI and VII of Larsell are involved in the regulation of saccadic eye movements2~.2~.2s. Electrophysiological investigations showed that neurons in this region were activated by visual (see ref. 3), auditory (see ref. 11), vestibular 26, extraocular muscle 5 and neck muscle afferents 7. Furthermore, neurons in lobule VII can be activated by tecto-olivocerebellar pathways in the cat 19.20. My previous paper3 described that in the rat visual afterents evoked climbing fiber responses in the medial region of posterior vermis in the ipsilateral side (lobule IV, VIII, IX and X), but not in lobule VII. Since numerous experiments show that the superior colliculus is implicated in the generation of saccadic eye movements 1~,27, the present experiment is undertaken to determine whether neurons in lobule VII are activated by tectal stimulation in the rat. It is shown that stimulation of the superior colliculus evokes climbing fiber responses in a wide area of the vermal region of lobule VII in the ipsilateral side. Twenty-five male albino rats (Wistar) were used. Experimental procedures were described elsewhere'. Briefly, the animals were anesthetised by chloral hydrate (200-400 mg/kg, i.p.), and 7 animals were artificially respirated after immobilization by muscular injection of tubocurarine chloride (Ameri-

zol; Yoshitomi, Osaka). After the animals were mounted on the stereotaxic apparatus, a burr hole was made at the midline (AS, L0) 23. The skull was removed, and the posterior cerebrum, cerebellum, and upper parts of the spinal cord were exposed. The exposed areas were covered with a mixture of vaseline and mineral oil. Body temperature was maintained by a heating pad. Concentric electrodes were placed at the optic disc on both sides. A bipolar metal electrode (1 mm separated) was placed at the optic nerve by perpendicular penetration through the cerebrum (A8, L0). Monopolar electrodes were placed in the intermediate to deep layers of the superior colliculus on both sides (A0.5-1.5, L0.5-2.0). Depth of the tectal electrodes were adjusted to evoke maximum responses in lobule VII. In 4 animals a bipolar electrode (1.5 mm separated) was placed at the vermal region of lobule VII. These metal electrodes were insulated except the tips, and their locations in the brain were identified histologically after experiments. For electrical stimulation rectangular pulses (0.2 ms in duration, 100-500 u A in amplitude) were derived from isolation units and delivered at 0.5-1.0 Hz. For recordings glass pipettes were filled with 2 M NaC1 solution saturated with fast green. The recording electrodes had electrical resistance of 5-8 MC2, which was connected to a neutralized preamplifier,

Correspondence: T. Akaike, D e p a r t m e n t of Physiology, Nagoya University School of Medicine. Nagoya 466, Japan. 0(/06-8993/85/$03.30 © 1985 Etsevicr Science Publishers B.V. (Biomedical Division)

37() then to A T A C 250 (storage oscilloscope, Nihonkoh-

larly penetrated in the vermal region ot lobule VII. Stimulation of the optic nerve or the optic disc of

den). Responses were photographed and analysed after experiments. Locations of paravermal veins

either side evoked no clear field potentials, even no

and of other veins surrounding each posterior lobule

clear mossy fiber activation of cerebellar cortical neurons. However, tectal stimulation evoked large,

were measured. Recording sites were marked by ejecting the dye, and identified histologically after

negative field potentials (latency. 7-11 ms) at certain

experiments. As described in the previous paper 3, stimulation of the optic disc evoked climbing fiber responses in the

depths, usually at two points, e.g., 0 . 1 - 0 . 2 mm and 0 . 6 - 0 . 7 mm deep from the cerebellar surface (Fig.

medial region of lobule IV, IX and X in the ipsilateral

tained at lower points than those at upper points.

side. Thus, the midline of the cerebellum was to be determined. A recording electrode was perpendicu-

These large negative field potentials were reversed

1). Larger field potentials were occasionally ob-

to positive potentials by change of depth of recording

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Fig. 1. Climbing fiber responses evoked in lobule VII of the posterior vermis by stimulation of the superior colliculus in an albino rat. A: dorsocaudal view of the exposed cerebrum, cerebellum and the upper part of the spinal cord. Recording sites in lobule VII were indicated by dots. B: the climbing fiber responses were superposed more than 10 times at each site. They were recorded transversely at 100 pm intervals in lobule VII (VII) at 0.6 mm deep from surface of the cerebellum. Recording sites were indicated by number (unit. 100pm) and by filled circles in the lower drawings. At the right edgerecording sites were moved rostrally at 200/~m. Deep layer of the superior colliculus (SC) was stimulated by a rectangular pulse (0.2 ms in duration. 300/~A in amplitude) on the right (R) or left (L) side. Note that the evoked field potentials distributed ipsitaterally to t .3 mm. and contrataterally to (~ 1-0.2 mm. The broken line indicates approximately the midline of the posterior vermis fsee text); pvv, paravermal vein.

371 sites at a distance of 0.1-0.2 mm. Histological examination identified that these reversal points corresponded to a boundary between the granular and the molecular layers. Amplitudes of the field potentials displayed a waxing and waning, even at an optimum rate (1 Hz) of stimulation. These evidence indicated that the field potentials were composed of climbing fiber responses of Purkinje cellsm. It was also confirmed by occasional intracellular recording from Purkinje cells, which showed large excitatory postsynaptic potentials characteristic of climbing fiber responses 10. The climbing fiber responses evoked by the tectal stimulation were explored throughout the posterior vermis. These were only observed in lobule VII. No responses were observed in lobules IV, VIII, IX or X. Ipsilaterally these field potentials distributed to 1.2-1.6 mm and contralaterally much smaller potentials were recorded to 0.1-0.2 mm (Fig. 1). It covered almost three quarters of the vermal region of lobule VII. In albino rats the tectal stimulation activated contralateral inferior olivary neurons. No clear field potentials were evoked in the ipsilateral side at symmetrical regions of the inferior olive in the present study. In the nucleus an area of recording field potentials evoked by the tectal stimulation (latency, 4 - 6 ms) almost precisely overlapped with an area of recording antidromic field potentials by stimulation of lobule VII (latency, 3 - 5 ms). Neurons in the area were not activated by stimulation of visual pathways. It suggests that inferior olivary neurons which are activated by tectal afferents do not receive visual afferents, but project in turn to lobule VII. Possibility of current spread from tectal stimulating electrodes to the visual and other pathways can be eliminated, because at the most effective site in a deep layer of the superior colliculus intensity of stimulus current was 50-100 u A , enough to evoke maximal responses at certain sites in the lobule. Furthermore, the pretectal region, a relay of the visual pathways 3, and other mesencephalo-olivary fibers projected only in the ipsilateral side 9,29. Bifurcated axons of trigemino-collicular and trigemino-olivary fibers terminated also in the same side 17. Inferior olivary neurons which were activated by tectal afferents were spatially separated from those which were activated by visual afferents. The latter is situated dorsally and caudally to the for-

mer. These tectal recipient zones seem to correspond to the medial accessory olive, subnucleus c. Recent anatomical investigations, using anterograde axonal transport methods, revealed that tecto-olivary projections were localized contralaterally in the medial accessory olive, subnucleus c in the rat 29, cat 31, rabbit 2s or subnucleus b in the primates 12-~3. Furthermore, Frankfurter et al. ~2 suggested that in the primate the superior colliculus was an exclusive source of olivary afferents to lobule VII of the posterior vermis. Jeneskog, using electrophysiological techniques, demonstrated that in the cat tectal stimulation evoked climbing fiber responses in a wide area (1.5-3.0 mm wide) of the midline region of lobule VII, but not in lobule Vlll ~'),2°. Kawamura and Hashikawa 2: described that in the cat zone A in lobule VII was specifically wider (1.0-1.5 mm) than those in other lobules (0.2-0.4 mm). These anatomical and electrophysiological observations were in good agreement with the present results. However, in the cat tectal stimulation evoked climbing fiber responses equally on both sides in the medial region of l o b u l e V I I 19,2°, whereas in the rat the responses were only in the ipsilateral side and in a wide area of vermal region of lobule VII, as shown above. Though some species differences are thus recognized, it is clear these are tecto-olivocerebellar (Iobule VII) projections (see ref. 8). It should be noted that tectoolivocerebellar projections localize in one lobule, and extend laterally, not longitudinally transversing several lobules as other olivo cerebellar projections 4,22, and that the pathway is quite different from visual olivocerebellar pathways~. By recent anatomical findings that neurons in posterior portions of the cerebellar medial nucleus projected to intermediate and deep layers of the superior colliculus <15,>, and that Purkinje cells in the posterior vermis projected to posterior regions of the medial nucleus ~5, it means that tecto-olivocerebellar pathways make a loop between posterior vermis of the cerebellum and the superior colliculus. Such loops are also known between the spinal cord and the anterior lobe z,-~(see ref. 18). Another line of evidence indicates that olivocerebellar pathways exert tonic inhibitory actions on Purkinje cells 25. Although we can only speculate about the physiological functions of such loops now, lobule VII may stand on a similar position for the superior colliculus as the anterior lobe stands for the spinal cord.

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